I've been looking for a clipping circuit for a while, but here's the catch - I'd like it to:
monitor the output, then, when it comes to within 5 volts of the rails, to round off the waveform, instead of the nice square top that SS amps give. That way, it'd sound tubey at lower volumes, then clip like a tube amp too.
Got any ideas on this?
Chris
monitor the output, then, when it comes to within 5 volts of the rails, to round off the waveform, instead of the nice square top that SS amps give. That way, it'd sound tubey at lower volumes, then clip like a tube amp too.
Got any ideas on this?
Chris
calculate the amp gain, then put in a "soft" pre-clip circuit that kicks in at a signal level that brings the amp within 5 volts of clipping (NAD amps did this). or do what Peavey and Fender do, put in an opto compressor that kicks in just below clipping and reduces the gain of the amp as you approach clipping (but this never distorts at all, just levels the gain off)
The traditional way to get close to this is described on this page: GM Arts - Guitar Amplifiers
The diodes in the feedback path do soft clipping, the ones on the output do the hard clipping. If you search you will find countless schemes of adding variability to this with fixed or variable resistors, for example. It's your safest bet, if you try to get too exotic the chances of actually finishing the project get slimmer and slimmer.
Thanks for the link. Some neat circuits in there. Might try some of them.
I was aware of clipping circuits for the pre stage, but I'd prefer it if there was one for the power amp section.
Anyway, this has probably gone a little OT.
Thanks
Chris
I was aware of clipping circuits for the pre stage, but I'd prefer it if there was one for the power amp section.
Anyway, this has probably gone a little OT.
Thanks
Chris
Well, that's kind of irrelevant for 2 reasons. The first is that you can apply these techniques wherever you want, i.e. the little opamp triangle in the schematics could be the input stage or the power opamp output stage. The second is that if you seem to be assuming that these SS circuits will react like tube circuits where clipping in the input stage and output stage are very different. At the risk of oversimplifying, that analogy simply doesn't hold.
Anybody interested in this subject could do worse than read the series of articles by Hartley Peavey on the Peavey site.
http://www.peavey.com/support/technotes/hartley/chapter_1.pdf
...that's a link to the first one, you can get them all by changing the URL to read chapter_2.pdf and so on. Possibly the most interesting is chapter_7.pdf which deals with power and OP transformer saturation which has a major influence on the final sound.
This is a 'must read' for anyone building instrument amplification, it gives a real justification for using tubes which IMO does not exist in the case of hi-fi, and assists in the avoidance of true mistakes, such as the over-specification of transformers which is commonly done with the best of intentions, but can actually result in a poorer sound for guitar.
w
http://www.peavey.com/support/technotes/hartley/chapter_1.pdf
...that's a link to the first one, you can get them all by changing the URL to read chapter_2.pdf and so on. Possibly the most interesting is chapter_7.pdf which deals with power and OP transformer saturation which has a major influence on the final sound.
This is a 'must read' for anyone building instrument amplification, it gives a real justification for using tubes which IMO does not exist in the case of hi-fi, and assists in the avoidance of true mistakes, such as the over-specification of transformers which is commonly done with the best of intentions, but can actually result in a poorer sound for guitar.
w
.... except it won't do what I want it to do. The ones shown clip all the time (until bypassed). What I want is something that only clips when the speaker output approaches the rail voltages. So, while they're pretty neat and would do nicely as a pedal effect, they won't protect the speaker from a square wave.
Perhaps I wasn't clear in my previous posts. I have added an output transformer to my SS amp. Now that it sounds nice at lower volumes, I'd like something to make it clip nicely. A compression circuit, as mentioned previously, is close, but I'd want it so that, instead of compressing the whole signal, it rounds off the wave-form as shown in one of the diagrams from your link. Compressing only the top 20% of the wave, if you like. This would make it sound more like a tube amp, because of the similar clipping characteristics.
Chris
You could go for a hybrid amp.
Cheaper and simpler than all valve.
Or build a SS amp but use a pedal on the front end to get a warm overdrive sound.
Look up soft limiters, they tend to limit like a valve.
There are countless ways to increase the voltage threshold of clipping. You can, e.g.
- put a resistive voltage divider in front of the clipping diodes (located in amp's feedback loop)
- use a "ladder" of multiple diodes and resistors
- use discrete zener diodes
- Reference the diodes to DC voltage insteaf of ground
- Fit the clipping diodes directly between the amp's inputs, instead of between input and output. They will thus react to the difference in input signals caused by clipping, and softly limit the input signal.
And so on.
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I think you are getting confused here. If you build something SS, you don't have to build anything special to clip at the rails, you get that for free 😀
Your latter point doesn't make sense to me either. If we ignore your special transformer coupling, then the ONLY way to take the speaker out of the hard clipping system is to move the clipping to the input stage. If we include the transformer coupling in, and you think that it is having some effect, what difference does where the clipping occurs make?
If you play mostly cleanly, not at 11 all the time at the master gain (clipping at tube outputs), then there is no real reason to think that tube amps are any better than SS. The sound here is mostly from the speaker and cabinet setup. The guitar/pickups and effects pedal are there for you too. I would copy the cabinet and exact driver/speaker setup mostly using a SS amp sized for a little more power, up to 2x more than the original. You could copy the tone control circuits and any pre effects as well.
Having a circuit round off the signal at the power amplifier stage or the preamp stage does not matter as long as they both are set up to operate before the onset of clipping. Once set up ahead of the power amp you would not change the gain of the amplifier past that point.
If you use a soft clipper circuit as suggested adjust the gain of the circuit watching the output of your amplifier. As you increase it to the point of squaring off the tops of your signal you would then back it off to where you would not see any squaring.
Mind you if you really blast the circuit with level you would get squaring as you would also in a tube amp. A hifi clean amp that is. It would be just as devoid of 'color' as a transistor amp until you clip them.
The rounding off of some tube amps are partly due to having a 'loose' power supply, transformer saturation, nonlinearities of the tube in the extremes, a host of other things.
Fender went the clean route in the 70's with their amps and other than EQ they did not really effect the sound much up to the point of them clipping. For guitar this series is the least popular by musicians. Mind you some like the clean sound.
If you use a soft clipper circuit as suggested adjust the gain of the circuit watching the output of your amplifier. As you increase it to the point of squaring off the tops of your signal you would then back it off to where you would not see any squaring.
Mind you if you really blast the circuit with level you would get squaring as you would also in a tube amp. A hifi clean amp that is. It would be just as devoid of 'color' as a transistor amp until you clip them.
The rounding off of some tube amps are partly due to having a 'loose' power supply, transformer saturation, nonlinearities of the tube in the extremes, a host of other things.
Fender went the clean route in the 70's with their amps and other than EQ they did not really effect the sound much up to the point of them clipping. For guitar this series is the least popular by musicians. Mind you some like the clean sound.
i was referring to using a soft clip circuit right before the power amp, after the preamp. when done correctly, this acts as a limiter that keeps the output transistors out of saturation and provides a soft clipped output waveform while doing so. it's very much like a tube amp's output stage clipping characteristic, but doesn't sag, so it won't sound exactly like a tube amp. the soft clip circuit can be adjustable so that the onset of soft clipping could be anywhere between 50 and 80% of full power, and the limiting bounds anywhere between 80 and 98% of the output stage's hard clipping level.
i just looked at the NAD 3120 amp's soft clip circuit, and it doesn't even use an op amp (others did), but uses a piecewise linear bounding circuit with a "mushy" response.
i just looked at the NAD 3120 amp's soft clip circuit, and it doesn't even use an op amp (others did), but uses a piecewise linear bounding circuit with a "mushy" response.
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replacement speaker for champs
The Jensen P8R 8" speaker is the heart & soul of small vintage practice amp
Original speaker for the Fender Champ
The Jensen 'P' Series speakers use Alnico magnets
resonant and pretty crappola response in the open back cab makes it what it is.
The Jensen P8R 8" speaker is the heart & soul of small vintage practice amp
Original speaker for the Fender Champ
The Jensen 'P' Series speakers use Alnico magnets
resonant and pretty crappola response in the open back cab makes it what it is.
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